3-46: Continuous fermentation and microfiltration cell recycle applied to a solids-free, high gravity lignocellulose hydrolysate

Efficient fermentation of lignocellulose hydrolysates to ethanol is limited by the amount of sugars and inhibitors released during pretreatment.  Low sugar concentrations lead to high distillation costs and inhibitors decrease the specific growth and ethanol production rates.  Low ethanol titers can be remedied by a sugars concentration step prior to fermentation, but inhibitor concentrations increase as well, leading to further reduction in the ethanol production rate.  Continuous fermentation with cell recycle increases the yeast concentration, leading to low steady state concentrations of metabolizable inhibitors (including the sugar substrate) and faster ethanol production rates.

The goal of this study was to investigate the fermentation of a concentrated hydrolysate in a single chemostat with tangential flow microfiltration cell recycle.  Using a kinetic model to account for the inhibitory effects of ethanol, acetic acid, HMF, and furfural, conditions that maximize the volumetric ethanol productivity for the S. cerevisiae D₅A fermentation were predicted.  The model predicts a productivity of 4.4 g/l/h at 95% cell recycling and 98% sugar utilization, a 5 fold improvement over low cell density batch fermentation.  High cell density batch experiments demonstrate an increase in ethanol production rates, but also indicate that high cell densities may affect intrinsic kinetics and cell yields.  A preliminary continuous run demonstrated successful operation for over 100 hours, reaching high steady state cell densities of over 27 g DW/l.  Strategies investigated to achieve higher cell densities and productivities while maintaining cell viability and reducing membrane fouling will be discussed.